Dual band photo-detection devices are increasingly used for selective sensing of light radiation in two or more wavelengths bands.
Conventional dual band photo-detection devices are, such as that disclosed in the article in Ref [1], generally configured to define two bi-polar photosensitive components (such as photodiodes and/or phototransistors) serially arranged back to back to one another (i.e. with their conduction directions being respectively opposite to one another). Another configuration of a photodiode based dual band photodetector is described for example in Ref [2].
Standard photodiode (or more generally a bi-polar photosensitive component, hereinafter referred to simply as photodiode or photodiode junction) is used herein to generally designate a component including a p-n or p-i-n photodiode junction (namely adjacent n-type and p-type semiconductor layers with possibly an intrinsic layer between them) defining a depletion region between the n-type and p-type semiconductor layers, in which the photon absorbing layer resides at least partially within the depletion region of the junction. In other words, in the standard photodiodes (photodiode junctions), the band gap in at least part of the depletion region matches the band of radiation wavelengths which is to be detected/absorbed, such that a photon absorbing layer for absorbing photons of the correct wavelengths is defined at least partially within (and typically also in regions adjacent to) the depletion region. When such a bi-polar photosensitive component (photodiode) is under forward bias voltage it is generally conductive, regardless of any absorption of photons. When the bi-polar photosensitive component is under reverse bias voltage, the photons absorbed in the photon absorbing layer excite carriers which are removed by the built-in voltage in the depletion region, due to the reverse bias voltage, thereby giving rise to a photocurrent flowing across the bi-polar photosensitive component.
Accordingly, in dual band photo-detection devices which include two serially arranged back to back photodiode junctions (bi-polar photosensitive components) sensitive to different wavelengths, the sensed wavelengths can be selectively controlled by the direction of the bias voltage (at each time setting the applied bias voltage such that one photodiode junction is under a reverse bias causing it to generate photocurrent in response to light absorbed thereby, and the other photodiode junction is under forward bias causing it to “short circuit” (i.e. to conduct easily) and thereby conduct the photocurrent from the reverse biased photodiode).
However, bi-polar photosensitive components such as photodiodes and phototransistors, in which at least part of the photon absorbing region is depleted (namely in which the photon absorbing region generally includes part or all of the depletion region), are deficient in terms of their reverse bias dark current which deteriorates their signal-to-noise ratio (SNR). The term “dark current” is commonly used in the art to define the current flowing in a photosensitive component during a total dark condition (no photons of the correct wavelength incident on the photodetector). The signal-to-noise ratio in photosensitive components, particularly those operable for sensing light in the infrared (IR) regime, is conventionally improved by cooling the component, in some cases down to very low temperatures close to 0° K. The means for cooling and maintaining such a low temperature in photosensitive components, however, are cumbersome and expensive, and in any case can only reduce the noise down to a limited value.
There are many applications in which the level of light which is required to be sensed is very low, and therefore the sensitivity of the photosensitive components (typically photodiodes and/or possibly phototransistors) in the photo-detection devices is a critical requirement. It is well known in the art that the signal-to-noise ratio which can be obtained from the photosensitive components is limited by the level of the “thermal noise”, which in turn is related to the temperature of the component.
The dark current in bipolar photo-detection components is generally composed of two main constituents. The first constituent, hereinafter referred to as “the diffusion” dark current, is due to the thermal excitation of carriers across the complete energy band gap of the photon absorbing material(s)/layer(s) of the photosensitive component, followed by diffusion of the minority carriers thus created to the depletion region. The second component affecting the level of the dark current is known as the “Generation-Recombination” current, hereinafter referred to as “G-R” dark current. A reverse bias voltage applied to the bi-polar photosensitive component activates these G-R centers (also known as Shockley-Read-Hall (SRH) traps) in the depletion region thereof (e.g. near the interface between p-type and n-type doped semiconductor materials in the component), because it increases the electric field in the depletion region thereby removing mobile carriers the moment they are created when an electron transfers in or out of a SRH trap, i.e. enhancing “Generation” and preventing “Recombination”. The effect is particularly strong for those SRH traps close to the middle of the band gap, when the energies between the trap and each of the conduction and valence band edges are nearly equal (i.e. approximately equal to half the band gap).
The level of diffusion current can be significantly reduced by cooling the photosensitive component, to reduce the amount of thermally excited charge carriers in the photon absorbing region.
As for the level of “G-R” dark current, although it can also be reduced by cooling, it is reduced at a slower rate than the diffusion current. This is because due to the G-R centers in the depletion region, the amount of thermal energy or “activation energy” needed to excite an electron (charge carrier) in the depletion region from the valence band to a SRH trap, or from a SRH trap to the conduction band, is approximately halved compared with the activation energy for the diffusion current process. As mentioned above, when electron-hole pairs are generated in this way, they are immediately removed by the electric field of the depletion region, thereby providing a strong driving mechanism for the G-R current. Thus the level of the G-R dark current is reduced by cooling at a slower rate than the diffusion current, due to its smaller activation energy. At low temperatures, where the level of the diffusion dark current is reduced sufficiently, the G-R dark current generally becomes the most dominant component of the dark current.
Indeed, another way to reduce the level of G-R dark current is to increase the band gap in the depletion region, so that the most effective SRH traps which are near the middle of the band gap in the depletion region are further in energy from the nearest conduction and valence bands, thereby increasing the activation energy of the G-R process and thus suppressing the G-R current.
However, in conventional bi-polar photosensitive components, such as photodiodes and phototransistors, the depletion region extends into the photon absorbing layer of the components and therefore the band gap of the photon-absorbing layer, which is a-priori designed to match the photon energy of the wavelength band to be absorbed/sensed by the component, is at least partially in the depletion region. This makes conventional bi-polar photosensitive components deficient in terms of the level of G-R currents provided thereby, especially at lower temperatures where G-R current becomes the dominant dark current noise component.
Also, in some infra-red sensing materials like Hg1-xCdxTe, a large positive offset in the conduction band (i.e. the conduction band is higher in energy for larger values of x) and a large negative offset in the valence band (i.e. the valence band is lower in energy for larger values of x) prevent a photodiode design with an abrupt change in composition (e.g. an abrupt change in the x-value) since this will block the free flow of minority carriers. To this end, in cases where the band gap can only be changed gradually in order to ensure that the bands remain smooth, the band gap in at least part of the depletion region is not very different from that in the photon absorbing layer and lies within the range of wavelengths to be detected. In such cases if there are G-R centers in the band gap they will contribute a significant G-R dark current noise in the detector.
As mentioned above, it is desirable to increase the band gap in the depletion region in order to suppress G-R processes therein. Recently, new types of unipolar semiconductor photo-detection components or photodetectors have been proposed in which the majority carriers in the photon absorbing layer are blocked by a barrier while the minority carriers can flow freely (see for example U.S. Pat. No. 8,004,012). These devices include a photon absorbing layer and barrier layer (with a much larger band gap) having the same doping polarity (e.g. n-type in XBn devices and p-type in XBpp devices, terminology defined in for example in Ref [4] and often shortened simply to XBn or XBp). In these devices, the same doping polarity in the photon absorbing layer and the barrier layer ensures that a depletion region is not formed anywhere within the photon absorbing layer (i.e. near the junction between it and the barrier layer) thereby suppressing the level of G-R current from the photon absorbing layer. In some cases the band gap of the barrier layer is made very large (e.g. using suitable hetero-structures), such that G-R processes in the barrier layer are greatly suppressed, therefore strongly suppressing the G-R current from the barrier layer which is always depleted. In such unipolar devices the dark current noise of the detector is very low (diffusion limited) and is mainly due to diffusion dark current which can be effectively controlled by cooling the component.
U.S. Pat. No. 8,004,012 also discloses (see FIG. 7a therein) a dual band unipolar photo-detection structure including first and second photon absorbing layers of the same doping polarity spaced from each other by a barrier layer, also with the same doping polarity.
In addition to the dark current, for pixilated photodetector devices, which comprise a lateral arrangement of pixels (plurality of photosensitive components/regions), noise may be introduced to the signals of the photosensitive components (pixels) due to an unwanted surface leakage current which causes cross conduction between the pixels, or parasitic conduction between the pixels and the common contact. These surface currents are often relatively insensitive to temperature and usually cannot be totally eliminated by cooling the device. Surface currents are most effectively suppressed by applying an appropriate passivation treatment to the exposed surfaces of the photodetector, however the effectiveness of this treatment depends on the materials used in the photodetector and in the passivation treatment.